"Sudden cardiac arrest" occurs when someone's heart stops beating unexpectedly. Each year, more than 45,000 Canadians have a cardiac arrest. A bystander can do three things to improve survival: Call 911,start chest compressions and apply a defibrillator. Together, these actions can increase survival by up to 800%. The problem is that bystanders to cardiac arrest only provide CPR in about 3 of every 10 cardiac arrest cases and AED use in about 3 of every 100 cardiac arrest cases. There are many people in the community who are trained and willing to provide help for cardiac arrest victims such as off-duty paramedics, fire fighters, nurses, etc. When a cardiac arrest occurs in the city, it is likely that one of these people is nearby, but unaware of the emergency. The PulsePoint smartphone application enables these people to be notified by the local emergency 911 service when there is a cardiac arrest near to them. It can be freely downloaded to several common types of smartphones. When there is a cardiac arrest emergency, all nearby PulsePoint users are sent an alert from the 911 service. When the phones receive the alert, they ring, vibrate and display a text message saying "CPR NEEDED". The user's current location and the exact location of the cardiac arrest are then displayed on a map. Nearby public access AEDs are also indicated on the map. The smartphone users can then go to provide chest compressions and use an AED while paramedics are on their way. A video at www.pulsepoint.org shows how this works. The objective of the investigators is to measure whether the PulsePoint smartphone application increases bystander CPR or AED use for victims of cardiac arrest outside the hospital. This project will happen in the City of Toronto. The investigators have a plan to get as many people as possible to download the application, focusing on health care professionals who know CPR. The investigators will set up a webpage that helps people download the software to their phone. The investigators will randomize 911 calls to have a PulsePoint alert sent or not. The investigators will use statistical analysis to measure whether sending an alert to a smartphone increases the chances of bystander resuscitation.